Referring to FIG. 1, there are shown an earth station called a hub 1 and several frames I, . . . I+R, I+R+1, of a communication channel 2 shared by a plurality of ground stations (not shown). The hub, which may be one of these stations, constitutes a master station that performs a function of monitoring an upstream link of said channel, for the transmission, from one ground station to another, of packets of information in time slots selected randomly. As shown in FIG. 2, the protocol of communication between the stations is the following: each frame, of t(f) seconds in length, comprises a given number K of message slots 21(1) to 21(K) and a particular slot called common minislot pool or CMP. Each message slot (or data slot) has a time duration of t(m) and is composed of a data part (or data message DM) and K minislots, and the CMP is also composed of K minislots but without any data part. The time durations of the minislots of each message slot are very small compared to t(m), and these minislots occur at the beginning of the message slot.
When a message has to be transmitted from a first station to a second one, this message has to wait until the beginning of the next frame, here called frame I. It is noteworthy that, (before frame I, all the stations had received from the hub a feedback message or signal indicating which slot is free for new messages or for messages which had sent successfully a transmission request. If there are indeed some available message slots, the emitting station chooses randomly one of these slots and transmits its message in the data part of this slot. Together with this message transmitted in the data part, an associated transmission request is placed in one of the minislots (randomly chosen) of the concerned message slot, in order to announce the reservation of said slot. If there are no available slot, this station puts a transmission request in one of the minislots of the CMP.
After a given propagation delay (for instance 0.27 seconds as indicated in FIG. 1, in the case of a link via a satellite), the hub receives all the messages and transmission requests that have been sent in the frame I. It then detects collisions in the data slots and calculates the set of slots that are available (or available slot set ASS) in the frame I+R+1 that just follows the frame during which all the stations have received from the hub the feedback messages (all the stations that have transmitted in frame I being assumed to receive their feedback before frame I+R+1). A slot (in said frame I+R+1 for instance) is said to be available (i.e. not reserved) if no request has announced a retransmission in this slot, or on the contrary reserved if one (and only one) request has announced such a retransmission. If more than one request announce a retransmission in the same slot, no successful transmission is possible and this slot is let free for new arrivals, while retransmission requests now occur in the CMP.
After having calculated the available slot set (ASS), i.e. the set of slots available for new messages, the hub broadcasts to all the stations both said ASS and the status of each message slot in the frame (empty, or available=E; used with success=S; collision=C):
(1) if a station has sent a message in a data slot and receives a feedback signal S (successful transmission), it exits the protocol; PA1 (2) if a station receives a feedback signal C (collision) or has sent a request in the CMP, it reads the ASS broadcasted by the hub: PA1 (a) if the reference number of the minislot does not correspond to any of the slots of the ASS, it means that this number corresponds to a reserved slot and that the request for retransmission has been successful: the station sends its message in this reserved data slot, and will then exit the protocol; PA1 (b) if the reference number of the minislot corresponds to a slot of the ASS, it means that the request has failed, and this slot is available for new messages, while the concerned station has to use the CMP for retransmitting its request.
Such a contention-based protocol, suited to low traffic conditions and well adapted to applications involving the generation of short constant length messages (for instance to interactivity applications such as user authentication for pay-per-view or multimedia data base consultation), is described for example in the U.S. Pat. No. 4,641,304. With this protocol, called ARRA for Announced Retransmission Random Access, as a signal has been placed in an accompanying, randomly choosen minislot (together with the message transmitted in the data slot) in order to announce the reservation of this corresponding slot in the subsequent frame if said message collides with another, messages having collisions in their data part will retransmit in the announced slot if the announcement does not encounter collision. In case both the data part and the announcement part suffer collisions, the station tries to put a new announcement in one of the minislots of the CMP. It is therefore clear that, while the probability of collision in the data part of a slot is limited to the first access trial (avoiding further collisions with other messages), collisions in announcement minislots can occur more than once, considering either the minislot field in each slot or the CMP at the beginning of each frame. Furthermore, at higher channel loading conditions, repetitive collisions occur in the CMP field, between stations having new messages to transmit and stations having suffered collisions in both their data and minislot fields. Since the number of minislots in the CMP is limited to K, collisions in this field are never resolved. The protocol saturates when doubly collided messages (i.e. suffering collisions in both the data field and the minislot field) are not authorized to access the available slots of a frame and these available slots are consequently wasted. The efficiency of this protocol, defined by its capacity or maximum normalized throughput, is then limited to a value of about 0.42 at most, which means that less than 50% of the data slots are successfully used.